1. Field of the Invention
The present invention relates to an optical sheet by which light between polarizers disposed in the form of crossed-Nicol can be cut off at wide-ranging azimuth angles so that a good-display-quality vertically oriented liquid-crystal display device which is excellent in viewing angle and contrast can be formed.
The present application is based on Japanese Patent Application No. 2000-284760, which is incorporated herein by reference.
2. Description of the Related Art
Even in the case where light between polarizers disposed in the form of crossed-Nicol was cut off in a normal-line (frontal) direction in which an axis of transmission and an axis of absorption functioned normally, there was heretofore a problem that light leakage occurred in a displaced viewing azimuth crossing the optical axis so that the intensity of leakage light increased gradually as the angle of oblique viewing increased. The problem was revealed as a disadvantage that display quality was deteriorated by reduction in display contrast due to light leakage in a viewing azimuth displaced from the optical axis when the polarizers were disposed on opposite sides of the liquid-crystal cell to function as a polarizer and an analyzer to thereby form a liquid-crystal display device.
In a vertically oriented (VA) liquid-crystal cell in which liquid-crystal molecules were oriented approximately vertically with respect to a cell substrate so that light was able to be transmitted with little change of the plane of polarization, when polarizers were disposed in the form of crossed-Nicol on opposite sides of the cell, light was apt to be cut off in a frontal (normal) direction of a display panel perpendicular to the cell substrate in an in operative mode having no external voltage applied to thereby make it possible to form good black display easily, in comparison with a TN liquid-crystal cell or the like in which liquid-crystal molecules were oriented horizontally with respect to a cell substrate so that display quality was apt to be deteriorated by light leakage owing to birefringence of the liquid-crystal cell at light transmission. However, the problem that contrast was deteriorated by light leakage in an oblique viewing azimuth displaced from the optical axis of the polarizers because of the aforementioned problem caused by the polarizers still remained, even if a phase retarder exhibiting refractive index anisotropy of nx=ny>nz as disclosed in Unexamined Japanese Patent Publication Sho. 62-210423 was used in combination with the vertically oriented liquid-crystal cell for compensating for birefringence of the liquid-crystal cell caused by oblique viewing.